Stents are elongate tubes that are used to prop open occluded or narrowed vessels or body lumens. Among other things, stents are often used to maintain the patency of the biliary tree, or common bile duct. FIG. 1 is a partial, cross-sectional view of a biliary system 2 showing the common bile duct 2a, the left hepatic duct 2b, the right hepatic duct 2c, the gall bladder 2d, the pancreas 2e and the duodenum 2f. 
Strictures or occlusions that develop in the upper common bile duct and/or the left and right hepatic ducts can interfere with the proper drainage of those ducts. FIG. 2 illustrates a partial cross-sectional view of the biliary system 2 having strictures 3 within the common bile duct 2a, the left hepatic duct 2b and the right hepatic duct 2c. One method of establishing proper drainage through the diseased ducts is to prop open the ducts by placing stents, such as self-expanding biliary stents, within the diseased ducts. Because of the branched configuration of the duct anatomy it is often necessary to place two or more stents in an overlying or side-by-side configuration.
However, currently available stent and introducer geometries are such that placement of a first stent often impedes placement of a second stent. FIG. 3 illustrates the problems associated with the prior art method of placing stents in the common bile duct 2a and the left and right hepatic ducts 2b, 2c. That is, placing stent 16 within the common bile duct 2a and the left hepatic duct 2b impedes subsequent access to the stricture in the right hepatic duct 2c. This prevents placement of a stent in the right hepatic duct 2c. 
FIG. 3A illustrates one problem encountered in the prior art by placing two stents sequentially. That is, once the first stent is deployed, it impedes insertion of the second introducer 20 used to deploy the second stent. An alternative to sequential deployment of the stents is simultaneous deployment. Simultaneous deployment, however, requires the side-by-side arrangement of two stent introducers within the working channel of an endoscope. Depending on the size of the stents to be placed and the limited size of the working channel of the endoscope, this option may be unworkable.
Consequently, there is a need for a self-expanding stent delivery system which overcomes the problems associated with prior art delivery systems. Specifically, there is a need for a self-expanding stent delivery system which allows the physician to sequentially place a first and second stent in the side branches and main lumen of a bifurcation.